Technical Field
The present invention relates to a method for producing a porous glass base material, by which a base material in which cracking of the base material during deposition and variation in the outer diameter occur little can be produced in the case when a large-sized porous glass base material is produced.
Background Art
As a method for producing optical fibers, a VAD process (a vapor phase axial deposition process) is well-known. The VAD process is a process for producing a porous glass base material composed of a core layer and a clad layer by disposing a plurality of burners in a reaction container, feeding a glass raw material gas such as silicon tetrachloride, a combustible gas such as hydrogen and a combustion-assisting gas such as oxygen to the respective burners, hydrolyzing the glass raw material in oxyhydrogen flame to form glass microparticles, and depositing the generated glass microparticles along a central axis of a starting element starting from the starting element, wherein the starting element rotates around its central axis as a rotational axis and is relatively drawn up with respect to the burners.
Specifically, for example, as shown in FIG. 1, in a reaction container 1, a first burner 3 for depositing a core, a second burner 4 for depositing a first clad deposit, and a third burner 5 for depositing a second clad are disposed in this order, toward the distance from the vicinity of the central axis of the starting element 2 fixed on a hang shaft 6, and toward the upper side from the perpendicularly lower side, and germanium tetrachloride for doping GeO2 is fed to the first burner 3 besides silicon tetrachloride. Glass microparticles are sprayed from the first burner 3, the second burner 4 and the third burner 5 to the thin starting element 2 having a diameter of about 20 mm, which is drawn up while being rotated, and as shown in FIG. 2, a porous glass base material is gradually thickened at the lower part of the starting element 2 and grown to a desired outer diameter of a diameter of about 180 mm (formation of a non-product tapered part 8), and deposited with retaining the deposition state stable (formation of a product cylindrical part 9), whereby a desired porous glass base material is produced.
In the case when the gas flow amounts at a steady state for forming the product cylindrical part 9 are introduced in the respective burners from the starting of the deposition, since the amounts of the gases are too much with respect to the thin outer diameter of the non-product tapered part 8, problems that the deposit efficiency is significantly decreased, and that the density excessively increases and thus bending and deformation in the base material occur. Therefore, a method for solving such problem by presetting a gas flow amount at an initial stage of deposition smaller than a gas flow amount at a steady state is disclosed in JP 06-015413 B2 and JP 06-017238 B2.